Graphene-Reinforced Ceramic Composites and Uses Therefor

ABSTRACT

The disclosure provides novel graphene-reinforced ceramic composites and methods for making such composite materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/560,463 filed Jul. 27, 2012, which claims priority to U.S.Provisional Patent Application Ser. No. 61/574,206 filed Jul. 29, 2011,each incorporated by reference herein in their entirety.

STATEMENT OF GOVERNMENT RIGHTS

This work was supported under grant number DMR0954110 and 0900188 fromthe National Science Foundation, and Award Number: N000140910928 fromthe US Office of Naval Research. The U.S. government has certain rightsin the invention.

DESCRIPTION OF RELATED ART

Structural ceramics are becoming increasingly relevant for hightemperature applications. For example, silicon nitride (Si₃N₄) is a hightemperature resistant ceramic (up to 1500° C.) and is also consideredthe most reliable structural ceramic due to the formation of aninterlocking microstructure of α-Si₃N₄ that is reinforced with longrod-like β-Si₃N₄ grains. This self-reinforced microstructure results ina high resistance to fracture due to the β-Si₃N₄ grains and highhardness due to the matrix of α-Si₃N₄ grains. However, Si₃N₄ is notwidely used due to its overall low toughness properties in comparison tometals.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides ceramic compositematerial comprising a graphene-reinforced ceramic material. In oneembodiment, the graphene is present in the graphene-reinforced ceramicmaterial at between about 0.02% to about 1.5% on a volume percentagebasis. In another embodiment, the graphene is present in the ceramiccomposite material as two or more graphene sheets. In a furtherembodiment, the graphene is homogenously dispersed throughout theceramic composite material. In another embodiment, the graphenecomprises graphene or one or more graphene sheets that are present atone or more grain boundaries of the ceramic material. In a furtherembodiment, the ceramic material comprises Si₃N₄. In a still furtherembodiment, the Si₃N₄ is predominately α-Si₃N₄.

In a second aspect, the present invention provides methods for making aceramic composite material, comprising

(a) combining graphene with a ceramic material; and

(b) densifying the combination to produce a graphene-reinforced ceramicmaterial.

In one embodiment, the graphene and the ceramic material are each mixedwith cationic surfactant prior to the combining step. In anotherembodiment, the graphene is present in the combination at between 0.5%to 1.5% on a volume percentage basis. In a further embodiment, thecationic surfactant comprises cetyltrimethylammonium bromide (CTAB). Inanother embodiment, the method comprises

(a) providing

-   -   (i) a solution of graphene mixed with a cationic surfactant,        wherein the solution has a pH of approximately 4; and    -   (ii) a solution of ceramic material mixed with a cationic        surfactant, wherein the solution has a pH of approximately 4;

(b) sonicating

-   -   (i) the solution of graphene under conditions suitable to        disperse the graphene in the solution to produce a dispersed        graphene solution; and    -   (ii) the solution of ceramic material under conditions suitable        to disperse the ceramic material in the solution to produce a        dispersed ceramic material solution;

(c) combining the dispersed graphene solution and the dispersed ceramicmaterial solution to produce a composite mixture;

(d) sonicating the composite mixture under conditions suitable todisperse the graphene and the ceramic material in the composite mixture;

(e) milling the composite mixture;

(f) evaporating water from the composite mixture to produce a compositeslurry;

(g) heat treating the composite slurry under conditions suitable toremove the cationic surfactant to produce a composite powder; and

(h) densifying the composite powder to produce the graphene-reinforcedceramic material.

In a still further embodiment, the densifying comprises exposing thecomposite powder to:

(i) rapid heating of approximately 100° C./minute up to a maximumtemperature of between about 1500° C. and about 1700° C. for betweenabout 2 minutes to about 5 minutes;

(ii) pressure application of approximately 35 MPa; and

(iii) electric current with a maximum pulse current of 3000 A and amaximum pulse voltage of 10 Volts.

In another embodiment, the densifying is carried out using spark plasmasintering.

BRIEF DESCRIPTION OF THE DRAWINGS

The results set forth herein, and the properties and characteristics ofthe composites provided by the disclosure, can be advantageouslyunderstood with regard to the drawings.

FIG. 1: GPL and GPL-Si₃N₄ powder particles. (a) Low and (b) highresolution SEM images of as-produced graphene platelets showing tightlypacked platelets containing crumpled sheets of graphene. (c) Low and (d)high-resolution SEM images after colloidal processing indicatingpartially exfoliated GPL mixed with well-dispersed Si₃N₄ particles. Theimages clearly indicate GPL decorated with Si₃N₄ particles; the Si₃N₄particles are well dispersed throughout the surface area of the sheets.

FIG. 2: Spark plasma sintering of Si₃N₄ matrix material. (a) The densityof Si₃N₄ is plotted as a function of sintering temperature for twodifferent times at temperature which are 5-minute hold and 2-minute holdat temperature. 100% of theoretical density for the matrix material isobtained at, ˜1650° C., for 2-minutes hold at temperature; (b) Thedensification profile for this high density part shows a steady increasein displacement with increasing temperature up to the final sinteringtemperature; (c) The X-Ray diffraction phase analysis of the matrixafter SPS shows that for the highest density part (1650° C.) we retain aphase composition that is approximately 100% α-Si₃N₄.

FIG. 3: SEM fracture surface images of Si₃N₄ and GPL nanocomposites. (a)Low and (b) higher magnification SEM images of the high-density, 100%α-Si₃N₄, monolithic ceramic matrix material showing an equiaxed andhomogeneous grain microstructure. (c) Low and (d) higher magnificationSEM images of the sintered and fractured, 1.0-vol % GPL-Si₃N₄nanocomposite. The small white arrows illustrate the location of GPL onthe fracture surface image. The large crack running through the (imagec) indicates presence of GPL along the grain boundary of the matrixmaterial. Corresponding (e) Low and (f) high magnification SEM images ofthe 1.5-vol % GPL-Si₃N₄ nanocomposite.

FIG. 4: Toughening mechanisms in GPL-Si₃N₄ nanocomposites. (a)Microhardness testing resulting in the creation of radial cracksstemming from the microhardness indent (inset image). Closer examinationof the radial cracks reveals GPL bridging the crack at severallocations, two of which are shown in this high resolution SEM image. (b)Further examination of the radial cracks indicates that they follow atortuous crack propagation path. (c) Fracture surface of the bulk sampleindicates the presence of three-dimensional toughening mechanisms forthe GPL-Si₃N₄ nanocomposite.

FIG. 5: Toughness characterization and Raman spectra for GPL-Si₃N₄composites. (a) Toughness of the monolith systematically increases withincreasing GPL vol %, from 0-1.5-vol % GPL. The toughness of themonolith is enhanced by ˜235% using ˜1.5 vol % GPL. b) Ramanspectroscopy was used to characterize the structure of the GPLs withinthe Si₃N₄ after SPS high temperature densification (1650° C. for2-minutes). Raman spectroscopy of the as-produced graphene shows thestarting material as platelets and that the as-received starting powderof Si₃N₄ is not Raman active. At 0.02-vol % GPL the SPS induces atransformation of the multi-layer GPLs into few or bi-layer graphenewhich is detected by the appearance of a new peak (G′ band) at ˜2624cm⁻¹. At 0.5 vol % GPL, Raman signature of nanodiamonds is observed,while at 1.0- and 1.5-vol % GPL we detect a mixture of few-layered andmulti-layer graphene.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the singular forms “a”, “an” and “the” include pluralreferents unless the context clearly dictates otherwise. “And” as usedherein is interchangeably used with “or” unless expressly statedotherwise.

All embodiments of any aspect of the invention can be used incombination, unless the context clearly dictates otherwise.

In a first aspect, the present invention provides novel ceramiccomposite materials comprising a graphene-reinforced ceramic material.As exemplified below, the inventors have discovered thatgraphene-ceramic composites exhibit significantly enhanced toughness ofstarting ceramic materials, and thus broadening their use in materialsand processes that require high temperatures (>1000° C.). Exemplarymethods for making the graphene-reinforced ceramic materials aredescribed below.

Graphene is an allotrope of carbon, with a structure of one-atom-thickplanar sheets of sp²-bonded carbon atoms that are densely packed in ahoneycomb crystal lattice. Graphene can be prepared by any suitabletechnique, including but not limited to the techniques described in theexamples that follow, or may be purchased from commercial suppliers,including but not limited to Graphene Industries (Manchester, UK). Inone embodiment, the graphene is provided in the form of grapheneplatelets (GPL), which can be obtained from a number of commercialsources. Alternatively, the GPLs can be prepared by any suitabletechnique, such as those described below. As used herein, “GPLs” arestacks of 2 or more (2, 3, 4, 5, 6, or more) graphene sheets withnanoscale thickness (averaging less than 2 nm thickness). GPLs are alsocommonly referred to a multilayer graphene sheets.

Any suitable ceramic material may be part of the graphene-reinforcedceramic material, including but not limited to boride-, carbide-,nitride-, oxide-based ceramics, Si₃N₄, ZrB₂, TaB₂, TiB₂, NbB₂, TiC, ZrC,NbC, HfC, TaC, and oxide compounds including but not limited to, Al₂O₃,and ZrO₂, each of which may further form ceramic composites with, forexample, SiC, B₄C, TaB₂, or TaSi₂. In one embodiment, the ceramicmaterial comprises or consists of Si₃N₄.

The graphene is present in the graphene-reinforced ceramic material atbetween about 0.02% to about 1.5% on a volume percentage basis (totalsolids basis). In a further embodiment, the graphene is present in thegraphene-reinforced ceramic material at between about 0.5% to about 1.5%on a volume percentage basis. Thus, in various embodiments, the grapheneis present in the graphene-reinforced ceramic material at between about0.02% to about 1.5%; 0.05% to about 1.5%; 0.1% to about 1.5%; 0.2% toabout 1.5%; 0.3% to about 1.5%; 0.4% to about 1.5%; 0.5% to about 1.5%;about 0.5% to about 1.4%; about 0.5% to about 1.3%; about 0.5% to about1.2%; about 0.5% to about 1.1%; about 0.5% to about 1.0%; about 0.5% toabout 0.9%; about 0.5% to about 0.8% about 0.5% to about 0.7%; about0.6% to about 1.5%; about 0.7% to about 1.5%; about 0.8% to about 1.5%;about 0.9% to about 1.5%; about 1.0% to about 1.5%; about 1.1% to about1.5%; about 1.2% to about 1.5%; or about 1.3% to about 1.5%; on a volumepercentage basis.

In one embodiment, the graphene is homogenously dispersed throughout theceramic composite material. In one embodiment, the graphene or graphenesheets are homogenously dispersed throughout the composite within thegrain boundaries of the ceramic matrix. In another embodiment, thegraphene is present in the ceramic composite material as one or more (1,2, 3, 4, 5, etc.) graphene sheets or GPLs. As discussed below, graphenesheets/GPLs help to improve toughness of composites containing them.

In another embodiment, at least a portion of the graphene in thecomposite is present at one or more grain boundaries of the ceramicmaterial. While not being bound by any specific mechanism, the inventorsbelieve that graphene present within grain boundaries helps to preventcracks in the composite materials from changing their propagation pathsin two-dimensional space and forces cracks to propagate inthree-dimensional space, thus greatly strengthening the composite. Suchan unexpected fracture resistance mechanism has not previously beenreported in ceramic matrix composites. In one embodiment, the graphenepresent at one or more grain boundaries of the ceramic materialcomprises one or more graphene sheets or GPLs. Based on the examplesthat follow, it is expected that the graphene sheet(s) or GPL wraparound the matrix grain boundaries, increasing contact area with thematrix and further increasing toughness of the composite.

In one embodiment, the ceramic material comprises or consists of Si₃N₄.In a further embodiment, the Si₃N₄ in the graphene-reinforced materialconsists predominately (i.e., >99%) α-Si₃N₄ grains. In anotherembodiment, the Si₃N₄ in the graphene-reinforced material consistssolely α-Si₃N₄ grains.

It will further be understood by those of skill in the art that thegraphene-reinforced ceramic material may be of any size or geometrysuitable for a given use and which can be accommodated by appropriatemethods for preparing the materials, such as those disclosed herein. Inone embodiment, the composite material is a nanocomposite material. Thedensity of the graphene-reinforced ceramic material may be of anysuitable density. In one embodiment, the density is between about 3.175g/cm³ and about 3.223 g/cm³. The hardness of the graphene-reinforcedceramic material may be of any suitable hardness. In one embodiment, thehardness is between about 18.6 GPa and about 30.7 GPa. The toughness ofthe graphene-reinforced ceramic material may be of any suitabletoughness. In one embodiment, the toughness is between about 2.56MPa^(−m10) and about 7.91 MPa^(−m10).

The graphene-reinforced ceramic material may be used in any structurefor which they are suitable. In various embodiments, thegraphene-reinforced ceramic material may be incorporated into anystructure suitable for use in, for example, aerospace vehicleapplications, medical device applications, structural and buildingapplications, electronics applications, and energy application.

In a second aspect, the present invention provides methods for making aceramic composite material, comprising

(a) combining graphene with a ceramic material; and

(b) densifying the combination to produce a graphene-reinforced ceramicmaterial.

This aspect of the invention can be used, for example, to prepare thegraphene-reinforced ceramic materials described herein. The methods areshown in the examples that follow to produce graphene-ceramic compositesexhibiting significantly enhanced toughness compared to starting ceramicmaterials, and thus broaden their use in materials and processes thatrequire high temperatures (>1000° C.).

The graphene can be prepared by any suitable technique, including butnot limited to the technique described in the examples that follow, ormay be purchased from commercial suppliers, including but not limited toGraphene Industries (Manchester, UK). In one embodiment, the graphene isprovided in the form of graphene platelets (GPL), which can be obtainedfrom a number of commercial sources. Alternatively, the GPLs can beprepared by any suitable technique, such as those described below. Asused herein, “GPLs” are stacks of 2 or more (2, 3, 4, 5, 6, or more)graphene sheets with nanoscale thickness (averaging less than 2 nmthickness).

The methods can be used to toughen any suitable ceramic material. Anysuitable ceramic material may be used, including but not limited toboride-, carbide-, nitride-, oxide-based ceramics, Si₃N₄, ZrB₂, HfB₂,TaB₂, TiB₂, NbB₂, TiC, ZrC, NbC, HfC, TaC, and oxide compounds includingbut not limited to, Al₂O₃, and ZrO₂, each of which may further formceramic composites with, for example, SiC, B₄C, TaB₂, or TaSi₂. Inanother embodiment, the ceramic material comprises or consists of Si₃N₄.In a further embodiment, the Si₃N₄ consists predominately (i.e., >99%)α-Si₃N₄ grains. In another embodiment, the Si₃N₄ consists solely α-Si₃N₄grains.

In one embodiment, the graphene and the ceramic material are each mixedwith cationic surfactant prior to the combining step. This step helps toproduce highly dispersed graphene and ceramic material slurries, bycreating a positive electrostatic repulsive force between the two phasesof the composite materials and with each other. This electrostaticrepulsion develops due to the net charge from the positive head group onthe surfactant molecules on the graphene and the ceramic material. Anysuitable cationic surfactant can be used, including but not limited tocetyltrimethylammonium bromide (CTAB). The amount of surfactant used isgenerally based on its dry weight and is generally used at aconcentration above the critical micelle concentration. In anon-limiting embodiment, approximately 1% wt:wt of CTAB is mixed withthe graphene platelets and with the ceramic material.

The amount of graphene combined with the ceramic material is betweenabout 0.02% to about 1.5% graphene:ceramic material on a volumepercentage basis (total solids basis). In a further embodiment, theamount of graphene combined with the ceramic material is between about0.5% to about 1.5% graphene:ceramic material on a volume percentagebasis. Thus, in various embodiments, the amount of graphene combinedwith the ceramic material is between about 0.02% to about 1.5%; 0.05% toabout 1.5%; 0.1% to about 1.5%; 0.2% to about 1.5%; 0.3% to about 1.5%;0.4% to about 1.5%; 0.5% to about 1.5%; about 0.5% to about 1.4%; about0.5% to about 1.3%; about 0.5% to about 1.2%; about 0.5% to about 1.1%;about 0.5% to about 1.0%; about 0.5% to about 0.9%; about 0.5% to about0.8% about 0.5% to about 0.7%; about 0.6% to about 1.5%; about 0.7% toabout 1.5%; about 0.8% to about 1.5%; about 0.9% to about 1.5%; about1.0% to about 1.5%; about 1.1% to about 1.5%; about 1.2% to about 1.5%;and about 1.3% to about 1.5%; on a volume percentage basis.

In a non-limiting embodiment, the method comprises:

(a) providing:

-   -   (i) a solution of graphene mixed with a cationic surfactant        (such as CTAB), wherein the solution has a pH of approximately        4; and    -   (ii) a solution of ceramic material mixed with cationic        surfactant (such as CTAB), wherein the solution has a pH of        approximately 4;

(b) sonicating:

-   -   (i) the solution of graphene under conditions suitable to        disperse the graphene in the solution to produce a dispersed        graphene solution; and    -   (ii) the solution of ceramic material under conditions suitable        to disperse the ceramic material in the solution to produce a        dispersed ceramic material solution;

(c) combining the dispersed graphene solution and the dispersed ceramicmaterial solution to produce a composite mixture;

(d) sonicating the composite mixture under conditions suitable todisperse the graphene and the ceramic material in the composite mixture;

(e) milling the composite mixture;

(f) evaporating water from the composite mixture to produce a compositeslurry;

(g) heat treating the composite slurry under conditions suitable toremove the cationic surfactant to produce a composite powder; and

(h) densifying the composite powder to produce the graphene-reinforcedceramic material.

Based on the teachings herein, it is well within the level of those ofskill in the art to determine appropriate conditions for mixing,sonicating, combining, milling, evaporating, heat treating, anddensifying. Exemplary such conditions are provided in the examples thatfollow.

In one embodiment, the densifying comprises exposing the compositepowder to:

(i) rapid heating of approximately 100° C./minute up to a maximumtemperature of between about 1500° C. and about 1700° C. for betweenabout 2 minutes and about 5 minutes;

(ii) pressure application of approximately 35 MPa; and

(iii) electric current with a maximum pulse current of 3000 A and amaximum pulse voltage of 10 Volts.

The maximum temperature is between about 1500° C. and about 1700° C.; invarious embodiments, the maximum temperature may be between about 1600°C. and about 1700° C.; between about 1650° C. and about 1700° C.;between about 1500° C. and about 1650° C.; between about 1500° C. andabout 1600° C.; and between about 1600° C. and about 1650° C.

In one embodiment, this densifying is carried out using spark plasmasintering (SPS). The advantages of using SPS to densify ceramicsinclude: (1) rapid heating rates (up to 600° C./min); and (2)simultaneous applied pressure (60-120 MPa). SPS simultaneously appliespressure and quickly pulses electric current through a graphite diecontaining the ceramic powders that are to be densified. The pulsedcurrent assists in densification upon applied pressure and relies oncreep and related mechanisms for densification and not the conventionalsintering methods that involve diffusion and mass transport of materialacross the grain boundaries during long periods of time at elevatedtemperatures. Exemplary and non-limiting specific SPS conditions areprovided in the examples that follow.

Those having skill in the art will recognize that the starting materialsand method conditions/parameters may be varied, the sequence of themethod steps altered, and additional steps employed to producecomposites encompassed by the present disclosure, as demonstrated by thefollowing examples. Starting materials can be obtained from commercialsources or prepared by well-established literature methods known tothose of ordinary skill in the art. This will sometimes require ajudgment to modify the order of the method steps or to select oneparticular process scheme over another in order to obtain a desiredcomposite of the disclosure.

The preparation of the composite materials of the disclosure isillustrated further by the following examples, which are not to beconstrued as limiting the disclosure in scope or spirit to the specificprocedures and composites described in them.

Examples Summary

The majority of work in graphene nanocomposites has focused on polymermatrices. Here we report for the first time the use of graphene toenhance the toughness of bulk silicon nitride ceramics. Ceramics areideally suited for high temperature applications but suffer from poortoughness. Our approach uses graphene platelets (GPL) that arehomogeneously dispersed with silicon nitride particles and densified, at˜1650° C., using spark plasma sintering. The sintering parameters areselected to enable the GPL to survive the harsh processing environment,as confirmed by Raman spectroscopy. We find that the ceramic's fracturetoughness increases by up to ˜235% (from ˜2.8 to ˜6.6 MPa-m^(/12)) at˜1.5% GPL volume fraction. Most interestingly novel tougheningmechanisms were observed that show GPL wrapping and anchoring themselvesaround individual ceramic grains to resist sheet-pullout. The resultingcage-like graphene structures that encapsulate the individual grainswere observed to deflect propagating cracks in not just two- butthree-dimensions.

Structural ceramics are becoming increasingly relevant for hightemperature applications.¹² In particular, silicon nitride (Si₃N₄) is ahigh temperature resistant ceramic (up to 1500° C.), and is alsoconsidered the most reliable structural ceramic due to the formation ofan interlocking microstructure of α-Si₃N₄ that is reinforced with longrod-like β-Si₃N₄ grains¹². This self-reinforced microstructure resultsin a high resistance to fracture due to the β-Si₃N₄ grains and highhardness due to the matrix of α-Si₃N₄ grains.¹² The combination of highhardness and toughness in one material composition and the ability totailor each property based on the microstructure is the principal reasonwhy Si₃N₄ is considered the leading structural ceramic. However, Si₃N₄is still not widely used in many elevated temperature (>1000° C.)applications due to its overall low toughness properties in comparisonto metals.

Conventional ceramic matrix composites (CMCs) use one-dimensional fibersas the reinforcement phase such as, carbon fibers¹³ or carbonnanotubes,¹⁴⁻¹⁵ and ceramic whiskers.¹⁶⁻¹⁷ To the best of our knowledgethere are currently no reports showing CMCs processed with graphenereinforcement for mechanical properties enhancement. A major reason whyprocessing graphene reinforced bulk ceramic composites has been limitedis the thermal stability limitations of graphene at high temperature.Ceramics, start to densify and sinter at temperatures >1000° C., andSi₃N₄ is usually sintered at, 1800° C., therefore, making it challengingto incorporate graphene which has low thermal stability at temperaturesin excess of ˜600° C.¹⁸ In order to determine the true effect ofgraphene on the toughness of Si₃N₄, and to eliminate the effect of theβ-Si₃N₄ grains on toughening, we chose to control the matrixmicrostructure to be approximately 100% α-Si₃N₄ (equiax grains). We didso using spark plasma sintering (SPS), which is a process that reducesthe time at temperature from hours to minutes over conventionalsintering methods, thus allowing accurate control of the α- to β-Si₃N₄conversion, and also limits thermally induced structural damage to thegraphene platelets (GPL) by avoiding long processing times at hightemperature.

Results

In order to obtain uniform, densified, microstructures of ournanocomposites we used colloidal processing methods to createhomogenously dispersed particle systems in aqueous suspension.¹⁹⁻²²Highly dispersed GPL/Si₃N₄ nanocomposite slurries, using 0.02, 0.5, 1.0and 1.5-vol % graphene (total solids basis) were processed using methodsbased on our previous work with single-walled carbon nanotube(SWNT)-Si₃N₄ ceramic nano-composites.²³ Bulk quantities of GPL used inthis study were produced by the rapid thermal expansion (>2000° C.min⁻¹) of graphite oxide.²⁴⁻²⁵ The platelets were comprised on averageof ˜3-4 graphene sheets with less than 2 nm thickness.²⁵ Elementalanalysis gave the carbon to oxygen ratio in GPL as ˜9.1 to 1, whichconfirmed that the majority of oxygen containing moieties were expelleddue to the thermal shock. FIG. 1 a-b show typical scanning electronmicroscopy (SEM) images of the as-produced GPL. The GPL areagglomerated, as seen in FIG. 1 a, before the colloidal processing step.In FIG. 1 b, the higher resolution SEM image shows the wrinkled surfaceof a few partially separated graphene sheets.

We have previously shown that the dispersion of SWNTs using a cationicsurfactant, cetyltrimethylammonium bromide (C₁₆TAB) occurs because thehydrophobic SWNTs are attracted to the hydrophobic tails of thesurfactant, resulting in SWNTs that are covered in positively chargedsurfactant molecules.^(23,26) We used CTAB as the dispersant throughoutthis study to disperse GPLs, assuming similar carbon surface chemistryas the SWNTs. The amount of surfactant used was based on its dry weightand was used at a concentration above the critical micelleconcentration. We used 1.0 wt % CTAB to GPLs and 1.0 wt % CTAB to Si₃N₄powders in order to create a positive electrostatic repulsive forcebetween the two phases of the composite materials and with each other.This electrostatic repulsion was developed due to the net charge fromthe positive head group on the surfactant molecules on the grapheneplatelets and the Si₃N₄ particles. FIG. 1 c and FIG. 1 d show SEM imagesof the GPL and Si₃N₄ powder mixtures after aqueous colloidal processing.FIG. 1 c shows GPL separated from each other amongst a uniformdispersion of Si₃N₄ particles. FIG. 1 d shows a higher resolution imageof a GPL decorated with individual Si₃N₄ particles, which are alsoenveloped within the GPL.

SPS is a relatively new high temperature powder consolidation methodthat has already been used to successfully create fully-denseceramics,²⁷⁻²⁹ nanoceramics,³⁰⁻³¹ and ceramic nanocomposites reinforcedwith carbon nanotubes.^(23, 32-35) The advantages of using SPS todensify ceramics are: (1) rapid heating rates (up to 600° C./min); and(2) simultaneous applied pressure (60-120 MPa). SPS simultaneouslyapplies pressure and quickly pulses electric current through a graphitedie containing the ceramic powders that are to be densified. The pulsedcurrent assists in densification upon applied pressure and relies oncreep and related mechanisms for densification and not the conventionalsintering methods that involve diffusion and mass transport of materialacross the grain boundaries during long periods of time at elevatedtemperatures.²⁸ For this study, we used SPS to densify our nanocompositepowders with precise control of the matrix micro-structure, and to limitthe time at temperature to minimize the possibility of structural damageto the GPL at high temperatures and pressures. The protocols used forSPS operation, and nancomposite and graphene preparation are provided inthe Materials and Methods Section.

Table 1 shows the SPS heating rate, time at temperature, hold time,percent theoretical density (% TD) and final material compositionobtained for monolithic Si₃N₄.

TABLE 1 Physical properties of S_(i3)N₄ monoliths densified using SPS.Starting SPS Heating Sintering Hold Applied % Final Material RateTemperature Time Load Theoretical Material Composition (° C./min) (° C.)(min.) (MPa) Density Composition α-Si₃N₄ 100 1500 5 35 87.6 100% α-Si₃N₄α-Si₃N₄ 100 1575 5 35 98.4 100% α-Si₃N₄ α-Si₃N₄ 100 1600 5 35 99.5 100%α-Si₃N₄ α-Si₃N₄ 100 1700 5 35 99.0  83% α-Si₃N₄ α-Si₃N₄ 100 1600 2 3593.8 >99% α-Si₃N₄ α-Si₃N₄ 100 1625 2 35 97.0 >99% α-Si₃N₄ α-Si₃N₄ 1001650 2 35 100.0 >99% α-Si₃N₄

The goal of this preliminary densification investigation was todetermine the SPS parameters that would result in creating nearly 100%α-Si₃N₄ while maintaining high density at the lowest possibletemperature and shortest amount of time at temperature. FIG. 2 a showsthe density plot for monolithic Si₃N₄ sintered from, 1500 to 1700° C.,using two different hold times of two- and five-minutes at temperature.The 5-minute density plot shows that the density increases withincreasing temperature from, 1500 to 1600° C. (99.5% TD), and thenremains relatively constant up to, 1700° C. (99.0% TD). The two-minutedensity plot shows that the effect of temperature on densification issignificantly greater when using a shorter hold time (keeping theheating rate constant) and results in 100% TD at ˜1650° C. FIG. 2 bshows the shrinkage displacement curve and the heating profile curvethat was measured during the SPS run for the high density monolithicpart sintered at, ˜1650° C. for 2-minutes. The displacement curve showssteady shrinkage displacement with increasing temperature up to ˜1650°C., which represents ideal densification behavior. FIG. 2 c shows theX-Ray diffraction (X'Pert MPD Pro, PANalytical, Westborough, Mass., USA)(Cu Kα) patterns for the monoliths sintered at various temperatures andhold times. Specifically, the XRD spectrum for the monolith sintered at1650° C., for only 2-minutes, confirms that SPS can be used to tailorthe matrix microstructure to ˜100% α-Si₃N₄, while achieving highdensity. The accurate and reliable control of the matrix microstructure(α-Si₃N₄) is important because we want to evaluate the effect of GPL asstructural reinforcement within a uniform and homogenous equiax grainmatrix microstructure. The XRD spectrum for the monoliths sintered at,1500, 1600, and 1700° C., for five minutes at temperature are also shownin FIG. 2 c with the percentage of each phase listed in Table 1. Asexpected, the phase content transitions from, 100% α-Si₃N₄ to β-Si₃N₄formation with increasing sintering temperature.

FIG. 3 a and FIG. 3 b are SEM images of two different fracture surfacesfor the same specimen of ˜100% α-Si₃N₄ (>99% T.D.). The grain size isestimated as ˜500 nm and the grains are uniform throughout the fracturesurfaces. FIG. 3 c and FIG. 3 d are SEM images of two different fracturesurfaces for the same 1.0-vol % GPL-Si₃N₄ nanocomposite. FIG. 3 c is thelower resolution image showing the α-Si₃N₄ grain matrix microstructure.It also indicates homogeneous dispersion of the GPL throughout thenanocomposite (as pointed out by the small white arrows directly labeledon the image). This specific image (FIG. 3 c) was taken at this areabecause of the interesting interactions between the large crack thatruns through the bulk of the specimen and the GPL at the center of theimage. First, we notice that the GPL is protruding out of the fracturesurface and it is a large platelet that runs along the grain boundariesof the matrix. The long continuous platelet of graphene does not appearto deflect the crack propagation path in-plane. However, the crack doesnot penetrate or puncture through the graphene platelet either.Therefore, we believe that the crack is arrested at the GPL and has tochange directions (i.e. undergo out-of-plane deflection) to negotiatethe GPL. Thus it appears that the GPL (which are anchored at the grainboundaries) prevent cracks from changing their propagation paths in theconventional two-dimensional spaces and forces such cracks to propagatein three-dimensional space. Such a fracture resistance mechanism hashitherto not been reported in conventional CMC systems. FIG. 3 d is anSEM image of a fracture surface at a different location of the samenanocomposite (1.0-vol % GPL-Si₃N₄) and shows how the GPL (at the centerof the image) is anchored securely within the grain boundaries of thematrix microstructure. The image also depicts smaller GPL dispersedthroughout the microstructure of the nanocomposite (pointed out by thesmall white arrows directly labeled onto the image). In comparison withthe monolith (FIG. 3 b) the nanocomposite microstructure is less angularand appears to exhibit less of a brittle fracture microstructure.

FIG. 3 e and FIG. 3 f are SEM images of the same fracture surface forthe 1.5-vol % GPL-Si₃N₄ nanocomposite. The bulk fracture surface issimilar to the 1.0-vol % GPL-Si₃N₄ nanocomposite. At lower magnification(FIG. 3 e) one can clearly see the graphene sheets pulled out of thefracture surface and in other regions they create a single layer (orpossible few layers) veil of graphene that drapes over the fracturesurface (top left corner) which makes them difficult to distinguish atfirst glance. The higher resolution SEM image in FIG. 3 f illustrates awall of graphene sheets that follows the grain boundaries of the matrix.Conventional fiber-reinforced ceramic toughening mechanisms such as,fiber pull-out, are commonly observed on fracture surfaces of bulk CMCs.For our system also we do observe the pull-out of the graphene sheetsthat are tucked and wrapped around the matrix grains. We expect that theenergy required to pull out a sheet is greater than that of a fiber dueto “sheet-wrapping” around the matrix grain boundaries and the increasedcontact area with the matrix.

In order to qualitatively quantify the effect of graphene concentrationon the toughness of the ceramic, we used microhardness testing to induceradial cracking from the corners of the indentation. These cracks arethen measured in length in order to calculate a toughness value usingthe Antis equation³⁶ (Equation 1). This equation uses the measured,hardness (H), applied load (P), modulus (E=300 GPa, as measuredpreviously for Si₃N₄), crack length (c_(o)), and a constant for Vickersproduced radial cracks in brittle ceramics (0.16) in order to calculatea toughness value.

$\begin{matrix}{K_{C} = {0.16\left( \frac{E}{H} \right)^{1/2}\left( \frac{P}{c_{o}^{3/2}} \right)}} & (1)\end{matrix}$

The Vickers hardness number (H) used to calculate the toughness valueswas measured using an applied load of 9.8 N in order to avoid formingradial cracks. An applied load of, 98 N, was used to create reproducibleradial cracks that were used to measure crack values (c_(o)) used in Eq.(1).

FIG. 4 a shows a representative microhardness indentation (inset image)of the 1.0-vol % GPL-Si₃N₄ nanocomposite. The area of the indent isapproximately 150 μm² and was created using a 196 N applied load. FIGS.4 a and 4 b are high-resolution images of the microhardness inducedradial cracks. FIG. 4 a shows crack deflection resulting in a branchedcrack structure. Probing within the cracks (FIGS. 4 a and 4 b) one cansee direct evidence of “sheet pull-out” and graphene sheets that arebridging the cracks, which are directly labeled on the images. FIG. 4 aalso shows two regions within the crack where it appears that the GPLsare necking down to a smaller cross-sectional area within the crackwake. FIG. 4 c, shows the bulk fracture surface for the 1.0-vol %GPL-Si₃N₄ nanocomposite. The GPL at the center of the image isprotruding out of the surface and follows the contours formed by thematrix grain boundaries. This fracture surface again illustrates theability of the GPL to block the in-plane propagation of the crackthereby forcing it to climb over the wall of graphene sheets. Such afracture surface is unexpected for a ceramic and suggests that thetwo-dimensional GPL promotes the deflection of cracks inthree-dimensions.

FIG. 5 a is a plot of the calculated toughness values for the GPL-Si₃N₄nanocomposites, shown as a function of GPL concentration from 0-, 0.02-,0.5-, 1.0-, and 1.5-vol % GPL. The plot shows a systematic increase intoughness with increasing GPL concentration from, ˜2.8 to ˜6.6MPa-m^(1/2). The increase in toughness over the monolith is as high as˜235% (i.e. three-fold increase in toughness for the 1.5-vol % GPL-Si₃N₄nanocomposite over the monolith). The performance of GPL is superior tosingle-walled carbon nanotube (SWNT) additives at the same filler volumefraction. For example in Ref 23 we reported fracture toughness of ˜4.71MPam^(1/2) for a 1.0-vol % SWNT-Si₃N₄ composite which is significantlylower than the values reported here (˜5.8 MPam^(1/2)) for GPL.Fractography analysis (FIGS. 3-4) indicates the presence of a variety oftoughening mechanisms for GPL including, sheet wrapping, sheet pull-out,two and three dimensional crack deflection, and crack bridging. Table 2summarizes the density, theoretical density, hardness, and toughnessvalues for each nanocomposite (0.02-, 0.2-, 1.0-, and 1.5-vol %GPL-Si₃N₄) and the monolith that were all sintered at 1650° C. (for2-minutes).

TABLE 2 Physical and mechanical properties of graphene-S_(i3)N₄nanocomposites. Starting Material Density % Theoretical HardnessToughness Composition (g/cm³) Density (GPa) (MPa-m^(1/2)) 0.00 vol %Graphene + 3.223 100.0 22.3 ± 0.84  2.8 ± 0.12 100.00 vol % S_(i3)N₄0.02 vol % Graphene + 3.204 99.5 21.2 ± 0.34  2.7 ± 0.14 99.98 vol %S_(i3)N₄ 0.50 vol % Graphene + 3.198 99.7 19.3 ± 0.69 5.21 ± 1.00 99.50vol % S_(i3)N₄ 1.00 vol % Graphene + 3.175 99.3 20.4 ± 0.37  5.8 ± 1.1899.00 vol % S_(i3)N₄ 1.50 vol % Graphene + 3.175 99.6 15.7 ± 0.61  6.6 ±1.31 98.50 vol % S_(i3)N₄

Raman study was also performed to confirm that the sheet-like structuresobserved in FIGS. 3-4 are GPL. FIG. 5 b shows a collection of individualRaman spectra for as-produced graphene, Si₃N₄ (after sintering), and0.02-, 0.5-, 1.0-, and 1.5-vol % GPL-Si₃N₄ nanocomposites (aftersintering). The as-received graphene shows two clear peaks at ˜1317 cm⁻¹(D-band) and ˜1582 cm⁻¹ (G-band); note that the G′ peak at ˜2624 cm⁻¹ isabsent which is typical of multilayer sheets, or platelet configurationof graphene.³⁷ At ˜0.02-vol % of graphene in the ceramic, the Ramansignature after SPS indicates a new peak at ˜2624 cm⁻¹ for the G′ (or 2Dband). This indicates the thinning of the multilayer graphene plateletsinto few-layer or possibly bi-layer graphene. Therefore, the hightemperatures and pressures associated with SPS can transform thestructure of graphene from multilayers into few-layered graphene.However, as we increase the concentration from 0.02- to 1 and 1.5-vol %GPL, the intensity of the G′ peak diminishes suggesting that thegraphene platelets are no longer being thinned as effectively at thehigher loading fractions. Another interesting observation is that theRaman signature for the ˜0.5-vol % GPL composite showed no peaks for theG and G′ bands. Instead a new peak at ˜1332 cm⁻¹ appears correspondingto crystalline diamond. Clearly further investigation is warranted tounderstand the effect of spark plasma sintering (i.e. pulsing directcurrent while under simultaneous pressure) on the thinning of GPL andthe possible conversion of GPL to nanodiamonds.

To summarize, we used aqueous colloidal processing methods to obtainuniform and homogenous dispersions of GPL and Si₃N₄ ceramic particlesprior to densification using SPS. After densification at ˜1650° C. wefound direct evidence of graphene in the nanocomposites using Ramanspectroscopy. The only exception being ˜0.5-vol % of graphene for whichcase we found that the graphene was possibly converted into thenanodiamond phase. At lower concentrations (0.02-vol % GPL) we convertedthe GPL into thinner bi-layer or few-layered graphene sheets using SPS.We measured an ˜235% increase in toughness for the nanocomposite overthe monolith using only 1.5-vol % addition of graphene. The SPSparameters were adjusted to ensure that we maintained the same matrixmicrostructure phase composition and high density (100% α-Si₃N₄with >99% T.D.) for all samples. Most interestingly, we observed somevery unexpected toughening mechanisms on the fracture surfaces of thenanocomposites. The GPL appear to be anchored or wrapped underneath thematrix grains and result in the formation of a continuous wall ofgraphene along the grain boundaries that arrests and forces cracks topropagate in not in just two- but in three-dimensions in order tocontinue to propagate through the material. Such fracture behavior inceramics has hitherto not been reported. Graphene nanofillerreinforcement could potentially be used to enhance toughness for a rangeof ceramic materials enabling their widespread use in high performancestructural applications.

Materials and Methods

Nanocomposite Powder Preparation:

Cetyltrimethylammonium bromide (C₁₆TAB) was procured from Sigma-AldrichCorp., St. Louis, Mo. Si₃N₄ powders with mean particle size of ˜0.77 μmmeasured by X-Ray absorption/sedimentation technique and average surfacearea of ˜7.7-m²/g measured by standard BET N₂ adsorption was obtainedfrom GS-44, Allied Signal Inc., Torrance, Calif. Appropriate amounts ofgraphene and Si₃N₄ powder were added to separate containers of DI-waterwith C₁₆TAB pre-dissolved and the pH was adjusted to ˜4 using nitricacid. The graphene and Si₃N₄ solutions were sonicated for 40 and 20minutes respectively at 22 watts using a horn sonicator, Misonix S-4000,Qsonica LLC, Newtown, Conn. The dispersed solutions were then combinedand sonicated for an additional 10 minutes (22 W) before ball millingfor 24 hours using alumina media. After processing the water wasevaporated at ˜100° C. and the dried nanocomposite slurries were heattreated in argon to remove the surfactant using a heating rate of ˜5°C./min up to ˜500° C. for one hour hold time.

Graphene Preparation:

Graphite oxide was prepared by oxidizing graphite flakes (averagediameter of ˜48 μm) in a solution of concentrated sulfuric acid(95-98%), concentrated nitric acid (68%), hydrochloric acid (36-38%) andPotassium chlorate (99.5%) for 96 hours²⁴⁻²⁵. Thermal reduction ofgraphite oxide into graphene platelets was achieved by placing thegraphite oxide powder (200 mg) in a 200 mm inner diameter, 1 m longquartz tube that was sealed at one end. The other end of the quartz tubewas closed using a rubber stopper. An argon inlet was then insertedthrough the rubber stopper. The sample was flushed with argon for ˜10min, and the quartz tube was quickly inserted into a tube furnace(Thermolyne 79300, Thermo Fisher Scientific Inc., USA) preheated to˜1050° C. and held in the furnace for ˜35 seconds.

Spark Plasma Sintering (SPS):

The SPS apparatus (SPS 10-3, Thermal Technology, Santa Rosa, Calif.,USA) used in this study was operated using a max pulse current of 3000 Aand max pulse voltage of 10V. The pulse cycle was 25 ms on and 5 ms offusing a heating rate of 100° C./min. An external pressure of 35 MPa wasapplied from the 5 MPa preload at 20 MPa/min to the warm-up temperatureof 600° C. (767 A). The powders were heated inside a graphite die thatwas lined with graphite foil in order to prevent reactions with the diecase. The control temperature was monitored using a single-color opticalpyrometer that was focused to within ˜6 mm of the powder sample withinthe die. A secondary temperature measurement was conducted using a Mosheath type C thermocouple located in the lower punch, ˜4 mm, from thebottom of the powder sample. The punch temperature was higher than thedie temperature by a measured difference of 100-150° C. at the targetdensification temperature. A vacuum of at least ˜3 Pa was achievedbefore application of power. After sintering the power is shut off atand pressure held at ˜35 MPa for five minutes while the furnace coolsnaturally at, ≈300° C./min. Density values were measured using theArchimedes method. The rule of mixtures was used to calculate thenanocomposite densities, based on volume fraction, using density valuesof, ˜1.0 and ˜3.22 g/cm², for GPL and Si₃N₄, respectively.

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It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art and areto be incorporated within the spirit and purview of this application andscope of the appended claims. All publications, patents, and patentapplications cited herein are hereby incorporated herein by referencefor all purposes.

We claim:
 1. A method for making a ceramic composite material,comprising (a) combining graphene with a ceramic material; and (b)densifying the combination to produce a graphene-reinforced ceramicmaterial.
 2. The method of claim 1, wherein the graphene and the ceramicmaterial are each mixed with cationic surfactant prior to the combiningstep.
 3. The method of claim 1, wherein the graphene is present in thecombination at between 0.5% to 1.5% on a volume percentage basis.
 4. Themethod of claim 2, wherein the cationic surfactant comprisescetyltrimethylammonium bromide (CTAB).
 5. The method of claim 4, whereinapproximately 1% wt:wt of CTAB is mixed with the graphene and with theceramic material.
 6. The method of claim 1, wherein the methodcomprises: (a) providing (i) a solution of graphene mixed with acationic surfactant, wherein the solution has a pH of approximately 4;and (ii) a solution of ceramic material mixed with a cationicsurfactant, wherein the solution has a pH of approximately 4; (b)sonicating (i) the solution of graphene under conditions suitable todisperse the graphene in the solution to produce a dispersed graphenesolution; and (ii) the solution of ceramic material under conditionssuitable to disperse the ceramic material in the solution to produce adispersed ceramic material solution; (c) combining the dispersedgraphene solution and the dispersed ceramic material solution to producea composite mixture; (d) sonicating the composite mixture underconditions suitable to disperse the graphene and the ceramic material inthe composite mixture; (e) milling the composite mixture; (f)evaporating water from the composite mixture to produce a compositeslurry; (g) heat treating the composite slurry under conditions suitableto remove the cationic surfactant to produce a composite powder; and (h)densifying the composite powder to produce the graphene-reinforcedceramic material.
 7. The method of claim 6, wherein the densifyingcomprises exposing the composite powder to: (i) rapid heating ofapproximately 100° C./minute up to a maximum temperature of betweenabout 1500° C. and about 1700° C. for between about 2 minutes to about 5minutes; (ii) pressure application of approximately 35 MPa; and (iii)electric current with a maximum pulse current of 3000 A and a maximumpulse voltage of 10 Volts.
 8. The method of claim 7, wherein thedensifying is carried out using spark plasma sintering.